I always thought he'd feel guilty and awful as he waited for Arthur to return.
Some self-indulgent people (like Queen Victoria) might go into mourning for half a century, but Merlin was not self-indulgent and it was more like a millennium and a half. I can't believe that he never changed his life-style, priorities, obsessions and immediate concerns in all that time. We saw a wanderer in old man's guise, but I choose to believe that was visual shorthand, not a fixed image that summed up 1500 years of life. *g*